Hot-formed previously welded steel part with very high mechanical resistance and production method

ABSTRACT

The invention relates principally to a welded steel part with a very high mechanical strength characteristics obtained by heating followed by hot forming, then cooling of at least one welded blank obtained by butt welding of at least one first and one second sheet consisting at least in part of a steel substrate and a pre-coating which is constituted by an intermetallic alloy layer in contact with the steel substrate, topped by a metal alloy layer of aluminum or aluminum-based alloy. 
     This welded steel part claimed by the invention is essentially characterized in that the metal alloy layer ( 19, 20 ) has been removed from the edges ( 36 ) in direct proximity to the weld metal zone ( 35 ), while the intermetallic alloy layer ( 17, 18 ) has been left in place, and in that over at least a portion of the length of the weld metal zone ( 35 ), the ratio between the carbon content of the weld metal zone ( 35 ) and the carbon content of the substrate ( 25, 26 ) of either the first or the second sheet ( 11, 12 ) having the higher carbon content (Cmax) is between 1.27 and 1.59. 
     The invention likewise relates to a method for the fabrication of a welded steel part as well as the use of this welded steel part for the fabrication of structural or safety parts for automotive vehicles.

This invention relates principally to a very high-strength hot-formedwelded steel part.

The invention likewise relates to a method for the fabrication of awelded steel part as well as the use of this welded steel part for thefabrication of structural or safety parts for automotive vehicles.

The prior art discloses methods for the fabrication of welded steelparts from steel blanks of different compositions and/or thicknessesthat are continuously butt-welded to one another. In a first knownfabrication mode, these welded blanks are cold-formed. In a second knownfabrication mode, these welded blanks are heated to a temperature thatmakes possible the austenitization of the steel and are then hot-formedand rapidly cooled in the forming die. This invention relates to thissecond fabrication mode.

The composition of the steel can be selected both to make possiblesubsequent heating and forming operations and to give the welded steelpart high mechanical strength, high impact strength and good corrosionresistance.

Steel parts of this type are used in particular in the automobileindustry, and more particularly for the fabrication of anti-intrusionparts, structural parts or parts that contribute to the safety ofautomotive vehicles.

Among the hot-formable materials that have the characteristics requiredfor the above-mentioned applications, coated steel sheet described inpublication EP971044 has in particular a carbon content of between 0.10%and 0.5% by weight and includes an aluminum-based metal pre-coating.This sheet is coated, for example by continuous dip coating, in a bathcontaining, in addition to aluminum, silicon and iron in controlledconcentrations. The subsequent heat treatment applied during ahot-forming process or after the forming and cooling carried out afterthis heat treatment makes it possible to obtain a martensiticmicrostructure that gives the steel part a high mechanical strengthwhich can exceed 1500 MPa.

A known method for the fabrication of welded steel parts consists ofprocuring at least two steel sheets as described in publication EP971044, butt welding these two sheets to obtain a welded blank,optionally cutting this welded blank, then heating the welded blankbefore performing a hot forming operation, for example by hot stamping,to impart to the steel part the shape required for its application.

One known welding technology is laser beam welding. This technology hasadvantages in terms of flexibility, quality and productivity compared toother welding technologies such as seam welding or arc welding.

During the welding operation, however, the aluminum-based pre-coatingconsisting of an intermetallic alloy layer which is in contact with thesteel substrate, topped by a layer of metal alloy, is diluted with thesteel substrate within the weld metal zone, which is the zone that is inthe molten state during the welding operation and which solidifies afterthis welding operation, forming the bond between the two sheets.

In the range of aluminum contents of the pre-coating, two phenomena canoccur.

In a first phenomenon, if the aluminum content in the weld metal zone islocally high, intermetallic compounds are formed, resulting from thedilution of a portion of the pre-coating inside the weld metal zone, andthe formation of an alloy which occurs during the subsequent heating ofthe welded joint before the hot forming step. These intermetalliccompounds are sites where incipient cracking is most likely to occur.

In the second phenomenon, if the aluminum content in the weld metal zoneis lower, the aluminum, which is an alphagene element in solid solutionin the matrix, prevents the transformation into austenite which occursduring the step preceding the stamping. Consequently, it is no longerpossible to obtain martensite or bainite during the cooling after thehot forming and the welded seam contains ferrite. The weld metal zonethen exhibits a hardness and mechanical strength which are less thanthose of the two adjacent sheets.

To prevent the first phenomenon described above, publication EP2007545describes a solution which consists of eliminating, at the level of theperiphery of the sheets destined to be subjected to the weldingoperation, the superficial layer of metal alloy, leaving the layer ofintermetallic alloy. The removal can be performed by brushing or bylaser beam. The intermetallic alloy layer is preserved to guarantee thecorrosion resistance and to prevent the phenomena of decarburization andoxidation during the heat treatment that precedes the forming operation.

However, this technology does not always make it possible to prevent thesecond phenomenon described above: although the dilution of the thinintermetallic alloy layer results in only a very slight increase in thealuminum content in the weld metal zone (less than 0.1%), theconjugation of local aluminum segregations and the potential combinationof boron in the form of nitride in the weld metal zone results in adecrease in the hardenability in this zone. Therefore, the criticalhardening rate is increased in the weld metal zone compared to the ratein the two adjacent sheets.

FIG. 1 illustrates the hardness observed in the weld metal zone (profile2) and in the base metal (profile 1), i.e. the neighboring steel sheet,after heating to 900° C., followed by hot stamping and cooling at avariable rate. The hardness of the base metal is the hardness obtainedin the case of a sheet described in publication EP971044, which containsin particular 0.22% C, 1.12% Mn and 0.003% B. The hardness of the weldmetal zone is the hardness observed when the welding is performed asdescribed in publication EP2007545.

Profile 1 indicates that the critical martensitic hardening rate of thebase metal is 27° C./second because any cooling rate greater than 27°C./second results in a hardness of the sheet on the order of 480 HV anda totally martensitic microstructure.

On the other hand, profile 2 shows that the martensitic criticalhardening rate of the weld metal zone is 35° C./s. Therefore, a coolingrate after hot stamping between 27° C./second and 35° C./second will notresult in a sufficient hardness and a fully martensitic structure inthis zone.

In addition, this increase in the critical hardening rate in the weldmetal zone is accompanied by unfavorable cooling conditions in this weldmetal zone during the hot forming.

In fact, it is possible that the weld metal zone may lose contactcompletely with the cold die during the cooling for the reasonsindicated below, considered independently or in combination:

-   -   if the two sheets are of different thicknesses, on account of        the “step” designed in the die to make possible the displacement        of the material during the forming    -   on account of a possible misalignment between the die and the        welded blank.

Therefore, on the basis of the information provided above, for a coolingrate of the welded blank of less than 35° C./second, the weld metal zoneexhibits a heterogeneous microstructure and a decrease in the mechanicalcharacteristics of the joint, which can render the welded steel partunsuitable for the intended applications, in particular for theautomobile industry.

Another known welding method applied to the sheets described inpublication EP971044 is described in publication EP1878531.

This method consists of creating a weld metal zone that exhibits therequired mechanical strength characteristics for the welding of twosheets previously cut by shearing which, on account of this type ofcutting, exhibit aluminum-based pre-coating deposits on their cut edges.

The welding method consists either of hybrid laser-TIG welding, i.e. alaser beam combined with an electric arc generated by a TIG weldingtorch (“Tungsten Inert Gas”) equipped with a non-fusible electrode, orhybrid laser-MIG (“Metal Inert Gas”) welding for which the welding torchis equipped with a fusible wire electrode.

However, the steel parts hot stamped after the welding operation usingthis method also exhibit mechanical brittleness at the level of the weldmetal zone.

In fact, regardless of the proportion of filler metal in the case oflaser-MIG welding, the mixing in the welded metal zone is not sufficientto prevent the formation of zones with a high concentration of aluminum,which results in the absence of formation of martensite at the level ofthe weld metal zone during cooling and thus insufficient mechanicalstrength.

To obtain a desired level of dilution, it is necessary to add largeamounts of filler metal, which on one hand creates problems melting themetal added by the welding with the metal to be welded, and on the otherhand a large excess thickness at the level of the weld metal zone whichis undesirable for the forming process and the resulting part to bewelded is unable to meet the quality standards in effect in theautomobile sector.

In this context, the object of this invention is a welded steel partthat has very high mechanical strength, i.e. greater than 1230 MPa,obtained by heating in the austenitic range followed by the deformationof at least one welded blank obtained by butt welding of at least twosheets consisting at least in part of a steel substrate and apre-coating which consists of an intermetallic alloy layer which is incontact with the steel substrate, topped by a layer of metal alloy whichis an aluminum or aluminum-based alloy.

A particular object of the invention is a welded steel part of the typedescribed above for which the prior deformation consists of hot formingand for which the mechanical strength of the weld metal zone is greaterthan that of the two welded sheets or of at least one of the two weldedsheets.

For this purpose, the welded steel part with very high mechanicalstrength characteristics obtained by the invention is obtained byheating in the austenitic range followed by hot forming, then cooling,of at least one welded blank obtained by butt welding of at least afirst and a second sheet which consist at least in part of a steelsubstrate and a pre-coating which is constituted by an intermetallicalloy layer in contact with the steel substrate, topped by a metal alloylayer of an aluminum or aluminum-base alloy, and is essentiallycharacterized in that the the metal alloy layer is removed from theedges in direct proximity to the weld metal zone resulting from thewelding operation and constituting the bond between the first and secondsheets, while the intermetallic alloy layer is retained, and in that,over at least a portion of the weld metal zone, the ratio between thecarbon content of the weld metal zone and the carbon content of thesubstrate of the first or second sheet, whichever has the highest carboncontent Cmax, is between 1.27 and 1.59.

The above mentioned characteristics of the welded steel part claimed bythe invention are translated by a fracture that occurs in the base metaland not in the weld metal zone when the weld joint is subjected to auniaxial tensile stress perpendicular to the joint.

The welded steel part claimed by the invention can also have theoptional characteristics described below, considered individually or inall possible technical combinations:

the ratio between the hardness of the weld metal zone and the hardnessof the substrate of the first or second sheet that has the higher carboncontent Cmax is greater than 1.029+(0.36 Cmax), where Cmax is expressedin per cent by weight.

the composition of the substrate of at least the first or the secondsheet includes the following elements, expressed in per cent by weight:

-   -   0.10%≦C≦0.5%    -   0.5%≦Mn≦3%    -   0.1%≦Si≦1%    -   0.01%≦Cr≦1%    -   Ti≦0.2%    -   Al≦0.1%    -   S≦0.05%    -   P≦0.1%    -   0.0002%≦B≦0.010%,        the balance being iron and unavoidable impurities from        processing.

the composition of the substrate of at least the first or the secondsheet includes the following, expressed in per cent by weight:

-   -   0.15%≦C≦0.4%    -   0.8%≦Mn≦2.3%    -   0.1%≦Si≦0.35%    -   0.01%≦Cr≦1%    -   Ti≦0.1%    -   Al≦0.1%    -   S≦0.03%    -   P≦0.05%    -   0.0005%≦B≦0.010%,        the balance being iron and unavoidable impurities from        processing.

the composition of the substrate of at least the first or the secondsheet includes the following, expressed in per cent by weight:

-   -   0.15%≦C≦0.25%    -   0.8%≦Mn≦1.8%    -   0.1%≦Si≦0.35%    -   0.01%≦Cr≦0.5%    -   Ti≦0.1%    -   Al≦0.1%    -   S≦0.05%    -   P≦0.1%    -   0.0002%≦B≦0.005%,        the balance being iron and unavoidable impurities from        processing.

the carbon content of the weld metal zone is less than or equal to 0.35%by weight.

the metal alloy layer of the pre-coating contains, expressed in per centby weight, between 8 and 11% silicon, between 2 and 4% iron, with theremainder of the composition consisting of aluminum and unavoidableimpurities.

the microstructure of the weld metal zone contains no ferrite.

the microstructure of the weld metal zone is martensitic.

said hot forming of the welded blank is performed by a hot stampingoperation.

the respective cut edges of the peripheral edges of the first and secondsheets destined to be subjected to the welding operation contain noaluminum or aluminum alloy, the presence of which can result from aprevious cutting operation of each of the first and second sheets.

The invention further relates to a method for the fabrication of thewelded steel part described above.

For this purpose, according to the method claimed by the invention, atleast a first and a second steel sheet are provided, consisting of asteel substrate and a pre-coating which consists of an intermetallicalloy layer in contact with the steel substrate, topped by a metallicalloy layer which is an aluminum or aluminum-based alloy, and in whichthis metal alloy layer is removed from at least one surface of a portionof a peripheral edge of each of the first and second steel sheetsdestined to be subjected to the welding operation, leaving in place theintermetallic alloy layer, and the aluminum or aluminum-base alloy, thepresence of which can result from a prior cutting operation of each ofthe first and second sheets, is removed from the respective cut edges ofthe peripheral edges of the first and second sheets destined to besubjected to the welding operation, then the first and second steelsheets are butt welded at the level of the respective peripheral edgesof these first and second steel sheets from which the layer of metalalloy has been removed by means of a laser source and by using a fillermetal wire on at least parts of the length of the welded zone, therebyobtaining a welded blank in which the carbon content of the weld metalzone resulting from the welding operation and constituting the bondbetween the first and second sheets is between 1.27 and 1.59 times thecarbon content of the substrate of the sheet that has the higher carboncontent, then said welded blank is heated to give it a totallyaustenitic structure in the welded metal zone, then said welded blank ishot formed and heated to obtain a steel part, then said steel part iscooled at a controlled rate to obtain the specified mechanical strengthcharacteristics.

The method for the fabrication of the welded steel part claimed by theinvention can also include the optional characteristics described below,considered individually or in all possible technical combinations:

the metal alloy layer is removed from the opposite faces of therespective peripheral edges of each of the first and second steelsheets, leaving the intermetallic alloy layer in place.

the width of the zone from which the metal alloy layer is removed at thelevel of the peripheral edge of the first and second sheets destined tobe subjected to the welding operation is between 0.2 and 2.2 mm.

the composition of the substrate of at least the first or the secondsheet includes the following, expressed in per cent by weight:

-   -   0.10%≦C≦0.5%    -   0.5%≦Mn≦3%    -   0.1%≦Si≦1%    -   0.01%≦Cr≦1%    -   Ti≦0.2%    -   Al≦0.1%    -   S≦0.05%    -   P≦0.1%    -   0.0002%≦B≦0.010%,        the balance being iron and unavoidable impurities from        processing.

the composition of the substrate of at least the first or the secondsheet includes the following, expressed in per cent by weight:

-   -   0.15%≦C≦0.4%    -   0.8%≦Mn≦2.3%    -   0.1%≦Si≦0.35%    -   0.01%≦Cr≦1%    -   Ti≦0.1%    -   Al≦0.1%    -   S≦0.03%    -   P≦0.05%    -   0.0005%≦B≦0.010%,        the balance being iron and unavoidable impurities from        processing.

the composition of the substrate of at least the first or the secondsheet includes the following, expressed in per cent by weight:

-   -   0.15%≦C≦0.25%    -   0.8%≦Mn≦1.8%    -   0.1%≦Si≦0.35%    -   0.01%≦Cr≦0.5%    -   Ti≦0.1%    -   Al≦0.1%    -   S≦0.05%    -   P≦0.1%    -   0.0002%≦B≦0.005%,        the balance being iron and unavoidable impurities from        processing.

during the welding step, the peripheral edges to be welded of the firstand second steel sheets are located at a maximum distance of 0.1 mm fromone another.

the linear welding energy of the laser source during the weldingoperation is greater than 0.3 kJ/cm.

the laser source is either of the CO₂ gas laser type, which delivers alinear welding energy greater than 1.4 kJ/cm, or of the solid statelaser type which delivers a linear welding energy greater than 0.3kJ/cm.

the welding speed is between 3 meters/minute and 8 meters/minute, andthe power of the CO₂ gas laser is greater than or equal to 7 kW and thepower of the solid state laser is greater than or equal to 4 kW.

the welding step is performed under helium and/or argon cover gas.

the helium and/or argon flow rate during the welding step is greaterthan or equal to 15 liters per minute.

the filler wire contains the following elements, expressed in per centby weight:

-   -   0.6%≦C≦1.5%    -   1%≦Mn≦4%    -   0.1%≦Si≦0.6%    -   Cr≦2%    -   Ti≦0.2%        the balance being iron and unavoidable impurities from        processing.

the filler wire contains the following elements, expressed in per centby weight:

-   -   0.65%≦C≦0.75%    -   1.95%≦Mn≦2.05%    -   0.35%≦Si≦0.45%    -   0.95%≦Cr≦1.05%    -   0.15% Ti≦0.25%        the balance being iron and unavoidable impurities from        processing.

the proportion of filler metal relative to the volume of the weld metalzone is between 12% and 26% and the welding speed is between 3 and 7meters per minute.

the pair consisting of the above proportion of filler metal relative tothe volume of the weld metal zone and the welding speed is within therange illustrated in FIG. 8.

the pair consisting of the above proportion of filler metal relative tothe volume of the weld metal zone and the welding speed meets thecombined requirements listed below:

-   -   the proportion of filler metal relative to the volume of the        weld metal zone is between 12% and 26% and    -   the welding speed is between 3 and 7 meters per minute, and    -   when the welding speed is greater than 3.5 meters per minute,        the pair consisting of the proportion of filler metal relative        to the volume of the weld metal zone (35) and the welding speed        is such that Y≦−3.86X+39.5, whereby Y designates the proportion        of filler metal expressed as a volume percentage and X        designates the welding speed expressed in meters per minute.

the proportion of filler metal relative to the volume of the weld metalzone (35) is between 14 and 16%, the helium and/or argon flow rate isbetween 13 and 17 liters per minute, the diameter at the point of impacton the sheet of the laser beam (30) is between 500 and 700 μm, and theextremity (32 a) of the filler wire (32) is at a distance from the pointof impact of the laser beam on the sheet of between 2 and 3 mm.

the cooling rate of the weld metal zone (35) during the hot forming stepis greater than or equal to the critical martensitic hardening rate ofthe weld metal zone (35).

Finally, the invention relates to the utilization of the steel partsdescribed above for the fabrication of structural or safety parts forvehicles, in particular automotive vehicles.

Other characteristics and advantages of the invention are portrayed indetail in the following description, which is presented exclusively byway of example and is in no way intended to be restrictive, withreference to the accompanying figures in which:

FIG. 1 presented above represents the comparative profile of thehardness of the base metal and of the weld metal zone as a function ofthe cooling rate during the hot stamping, for a welded steel part of theprior art,

FIG. 2 is a schematic illustration of the sheet used in theimplementation of the method claimed by the invention,

FIG. 3 is a schematic illustration of the beginning of the weldingoperation of the method claimed by the invention,

FIG. 4 is a schematic illustration of the end of the welding operationof the method claimed by the invention,

FIG. 5 illustrates the profile of the mechanical tensile fracturestrength of the weld metal zone, whereby the stress is exertedperpendicularly in relation to the welded joint, as a function of thepercentage of filler metal in the weld metal zone during the methodclaimed by the invention, and for two different cooling rates during thehot stamping,

FIG. 6 illustrates the location of the fracture, either in the basemetal or in the weld metal zone, as a function of the ratio between thecarbon content of the weld metal zone and the carbon content of the basemetal,

FIG. 7 is a graph that illustrates one example of a micro-hardnessprofile (hardness under a load of 200 g) of a welded steel partfabricated from two sheets of different thickness and stamped accordingto the invention and of the zone adjacent to the weld metal zone, and

FIG. 8 is a graph illustrating the optimum operating limit conditions ofthe method claimed by the invention in terms of percentage of fillermetal and welding speed.

FIG. 9 illustrates the variation of toughness in the weld metal zone asa function of the temperature for different carbon contents.

In the method claimed by the invention, two sheets coated by immersionin a bath of molten aluminum according to a method called continuous“dip coating” as described in publication EP971044 are provided. Theterm sheet is used in a broad sense as any strip or object obtained bycutting from a strip, coil or sheet.

The aluminum bath which is the object of the dipping operation can alsoinclude from 9 to 10% silicon and from 2 to 3.5% iron.

The steel constituting the steel substrate of the sheets exhibits thefollowing composition, expressed in per cent by weight:

-   -   0.10%≦C≦0.5%    -   0.5%≦Mn≦3%    -   0.1%≦Si≦1%    -   0.01%≦Cr≦1%    -   Ti≦0.2%    -   Al≦0.1%    -   S≦0.05%    -   P≦0.1%    -   0.0002%≦B≦0.010%,        the balance being iron and unavoidable impurities from        processing.

Preferably, the composition of the steel will be as follows:

-   -   0.15%≦C≦0.4%    -   0.8%≦Mn≦2.3%    -   0.1%≦Si≦0.35%    -   0.01%≦Cr≦1%    -   Ti≦0.1%    -   Al≦0.1%    -   S≦0.03%    -   P≦0.05%    -   0.0005%≦B≦0.010%,        the balance being iron and unavoidable impurities from        processing.

Even more preferably, and in accordance with the following description,the composition of the steel will be as follows:

-   -   0.15%≦C≦0.25%    -   0.8%≦Mn≦1.8%    -   0.1%≦Si≦0.35%    -   0.01%≦Cr≦0.5%    -   Ti≦0.1%    -   Al≦0.1%    -   S≦0.05%    -   P≦0.1%    -   0.0002%≦B≦0.005%,        the balance being iron and unavoidable impurities from        processing.

The sheets to be welded to one another can be of identical or differentcompositions.

The coating, which will be called the “pre-coating” at this stage in thefollowing description, exhibits the following characteristics resultingfrom the immersion of the sheet in the aluminum bath: with reference toFIG. 2, the pre-coating 3 of the sheet 4 has two layers 5, 7 ofdifferent types.

First, an intermetallic alloy layer 5 of the AlSiFe type is in contactwith the surface of the steel substrate 6 of the sheet 4. Thisintermetallic alloy layer 5 results from the reaction between the steelsubstrate 6 and the aluminum bath.

In addition, this intermetallic alloy layer 5 is topped by a metal alloylayer 7 which forms a surface layer of the pre-coating 3.

The pre-coating 3 is present on the two opposite faces 8 a, 8 b of thesheet 4.

In the method claimed by the invention, the metal alloy layer 7 isremoved at the level of the periphery 9 of the sheet 4 which is destinedto undergo the subsequent welding operation.

In FIG. 2, only the upper surface 8 a is the object of this removal, butthe metal alloy layer 7 will advantageously be removed peripherally atthe level of the two opposite faces 8 a, 8 b of the sheet 4.

The intermetallic alloy layer 5 therefore remains at the level of theperiphery 9 of the sheet 4 which is destined to undergo the weldingoperation.

The ablation of the metal layer 7 can be performed by a brushingoperation because the metal layer 7 which is removed has a hardnesswhich is less than the hardness of the intermetallic alloy layer 5 whichremains.

A technician skilled in the art will understand how to adapt theparameters relating to the brushing to make possible the removal of themetal layer 7 on the periphery 9 of the sheet 4.

It is also possible to remove the metal alloy layer using a laser beamdirected toward the periphery 9 of the sheet 4.

The interaction between the laser beam and the pre-coating 3 causes avaporization and an expulsion of the metal alloy layer 7.

The width over which the metal alloy layer 7 at the level of theperiphery 9 of the sheet 4 is removed is between 0.2 and 2.2millimeters.

In addition, the intermetallic alloy layer 5 that remains at the levelof the periphery 9 of the sheet 4 is on the order of 5 μm thick.

These two modes of ablation (brushing and laser) of the metal alloylayer are the subject of publication EP2007545.

The previous cutting operations of the sheet 4, as well as the operationof removing the metal alloy layer 7 as described above can involve aportion of the pre-coating 3 at the level of the cut edge 10 of theperiphery 9 of the sheet 4 destined to be the object of the weldingoperation. Therefore there are traces of aluminum or aluminum alloy atthe level of this cut edge 10.

According to the method claimed by the invention, these traces ofaluminum or aluminum alloy at the level of the cut edge 10 of the sheet4 are also removed by brushing prior to the welding operation.

With reference to FIG. 3, a first sheet 11 and the second sheet 12, eachhaving a respective substrate 25, 26, and each having on theirrespective opposite faces 13 a, 13 b; 14 a, 14 b a pre-coating 15, 16consisting of an intermetallic alloy layer 17, 18 topped by a metalalloy layer 19, 20, are placed end to end according to conventionallaser welding techniques by contact between their respective peripheries21, 22, on which on one hand the metal alloy layer 19, 20 has beenremoved at the level of their opposite faces 13 a, 13 b; 14 a, 14 b, andon the cut edges 23, 24 from which the pre-coating 15, 16 depositedduring the shearing operation has also been removed.

The maximum distance between the respective cut edges 23, 24 of the twosheets 11, 12 is 0.1 mm, whereby the placement of this clearance betweenthe cut edges 23, 24 of the two sheets 11, 12 promotes the deposit ofthe filler metal during the welding operation.

As illustrated in FIG. 3, the welding operation according to the methodclaimed by the invention consists of a laser beam 30 directed at thelevel of the junction between the two sheets 11, 12, combined with afiller wire 32 that melts at the point of impact 31 of the laser beam.The welding method in question is therefore laser welding with fillermetal.

The laser source used must be high-powered and can be selected fromamong a laser CO₂ gas type laser source with a wavelength of 10micrometers or a solid state laser source with a wavelength of 1micrometer.

On account of the thickness of the two sheets 11, 12 which is less than3 mm, the power of the CO₂ gas laser must be greater than or equal to 7kW while the power of the solid state laser must be greater than orequal to 4 kW.

The diameter of the laser beam at the point of its impact on the sheetsmust be approximately 600 μm for both types of laser source.

Finally, the extremity 32 a of the filler wire 32 must be locatedapproximately 3 mm from the point of impact P of the laser beam 30 onthe junction between the sheets 11 and 12 for a solid state laser sourceand approximately 2 mm from the laser beam 30 for a CO₂ gas laser typelaser source.

These conditions make it possible to obtain a complete melting of thefiller wire 32 as well as a satisfactory mixing with the steel substrateat the level of the weld.

In addition, these powers will make it possible to use a welding speedsufficient to prevent the precipitation of boron nitrides and/or othersegregation problems.

The filler wire must meet two requirements:

first, the quantity of metal added by this filler wire 32 must be suchthat the laser source is able to melt it in its entirety and to producea relatively homogeneous mixture at the level of the weld. In addition,the quantity of metal added must not result in an overthickness of theweld by more than 10% in relation to the lowest thickness of the twosheets if the latter are not the same thickness, in accordance with thequality standards in force in the automobile industry.

the composition of the filler wire must also make it possible, incombination with the other parameters of the welding process, to obtaina weld, the mechanical strength characteristics of which are comparable,after hot forming and cooling, with the mechanical strengthcharacteristics of the first 11 and second 12 welded sheets.

Finally, during the welding step, cover gas protection must be providedto prevent the oxidation and decarburization of the zone which is beingwelded, to prevent the formation of boron nitride in the weld metal zoneand potential cold cracking phenomena caused by the absorption ofhydrogen.

This cover gas protection is achieved by using helium and/or argon.

With reference to FIG. 4, the welding operation leads to the formationof a weld metal zone 35 at the junction between the two sheets 11, 12which subsequently solidifies, thereby forming the weld. The term “weldmetal zone” is used to identify this weld even after solidification ofthis weld metal zone 35.

Measures can be taken for the parts that undergo a less rapid localcooling during the hot forming to add a filler wire only in certainportions of the length of the weld metal zone and not to add the fillermetal wire in the remaining joints.

The welded blank 37 resulting from the welding operation therefore has aweld metal zone 35 that does not contain intermetallic alloy because ofthe prior removal of the metal alloy layer 19, 20 as explained above.

In addition, as illustrated in FIG. 4, the edges 36 in direct proximityto the weld metal zone 35 are free of the metal alloy layer 19, 20 onaccount of the fact that the width of the weld metal zone 35 is lessthan the width of the welding zone which does not include a metal alloylayer 19, 20.

Although FIG. 4 illustrates the simple case of a welded blank fabricatedfrom a first sheet 11 and the second sheet 12, it is possible in themethod claimed by the invention to use a larger number of sheets whichare welded to one another.

The welded blank 37 thereby obtained is then subjected to a heatingprocess to obtain an austenitic transformation in all of the parts ofthis blank. This blank is then hot formed, preferably by hot stamping.This step is followed by a cooling conducted by contact in the stampingdie at a cooling rate which is discussed below, and results in a weldedsteel part.

In the following description, the reference to a welded steel partrefers to the finished piece following the hot stamping of the weldedblank, the fabrication of which is described above.

For a type 22MnB5 steel (C=0.20-0.25%, Mn=1.1-1.35%, Si=0.15-0.35%,A1=0.020-0.060%, Ti=0.020-0.050%, Cr=0.15-0.30%, B=0.002-0.004%, thecontents being expressed in per cent by weight and the balanceconsisting of iron and the impurities resulting from processing), table1 below presents the conditions of the welding method used to fabricatea welded steel part for which the hardness of the weld metal andhot-stamped zone is at least equal to the hardness of one or the otherof the two sheets 11, 12.

These conditions are indicated in terms of welding speed, volumepercentage of the filler metal in relation to the weld metal zone andthe chemical composition of the filler wire expressed in per cent byweight. The tests that were conducted to determine these boundaryconditions were performed with a CO₂ gas laser source with a powergreater than 7 kilowatts and a solid state laser source with a powergreater than 4 kilowatts under a helium and/or argon cover gas at a flowrate greater than 15 liters/minute.

TABLE 1 Welding Proportion Composition of the filler wire - speed offiller % by weight (m/min) metal (%) C Mn Si Cr Ti Minimum 3 10 0.6 10.1 0 0 Maximum 8 26 1.5 4 0.6 2 0.2

In the framework of another example, tests were conducted with a fillerwire having the composition indicated below, in per cent by weight:C=0.7%, Si=0.4%, Mn=2%, Cr=1% and Ti=0.2, the remainder consisting ofiron and impurities resulting from processing.

The tests that were conducted to determine these boundary conditionswere performed with a CO₂ gas laser source with a power greater than 7kilowatts and a solid state laser source with a power greater than 4kilowatts under a helium and/or argon cover gas at a flow rate greaterthan 15 liters/minute. All the results obtained and presented below aresimilar, regardless of the laser source used.

With reference to FIG. 8, the appearance of the weld metal zone and thequality of the mixing of the filler wire and the molten metal areexamined for different percentages of filler metal and welding speeds.

For the experimental points identified as references 40 and 41, theresults in terms of dilution and surface appearance of the weld metalzone are satisfactory, while for the experimental points identified as42, the results are not satisfactory.

FIG. 5 illustrates the tensile fracture strength of the hot-stampedwelded steel part as a function of the percentage of filler metal in theweld metal zone for two cooling rates of 30 and 50° C. per second.

The experimental points identified as reference 43 correspond to acooling rate of 30° C. per second and the experimental points identifiedas reference 44 correspond to a cooling rate of 50° C. per second. Thesetwo rates correspond respectively to an efficient extraction of heatthanks to tight contact between the part and the press die (50° C. persecond) and to a less tight contact on account of a lower closingpressure and/or a difference in thickness between the sheets to bewelded (30° C. per second).

When the hot stamped welded blanks are cooled at a rate of 50° C. persecond, the tensile strength is between 1470 and 1545 MPa and thefracture occurs in the base metal.

When the hot stamped welded blanks are cooled at a rate of 30° C. persecond, and when the volume proportion of the filler metal is between4.3 and 11.5%, the fracture occurs in the weld metal zone and themechanical tensile strength is between 1230 and 1270 MPa.

On the other hand, when the hot stamped welded blanks are cooled at arate of 30° C. per second, and when the volume proportion of fillermetal is 14.7%, the fracture occurs in the base metal with a mechanicalstrength of 1410 MPa.

Therefore, a proportion of filler metal greater than 12% makes itpossible to systematically obtain a fracture outside the welded joint,both in the efficiently cooled zones in the hot stamped part and in theless efficiently cooled zones.

FIG. 6 illustrates the location of the fracture, either in the basemetal as indicated on step 45, or in the weld metal zone indicated onstep 46, when the welded joints are subjected to a uniaxial tensileforce perpendicular to the seam, as a function of the ratio between thecarbon content of the weld metal zone and the carbon content of the basemetal, starting from the experimental points 43, 44 presented withreference to FIG. 5 and identified respectively as 43 a and 44 b in FIG.6.

It has been shown that when this ratio is greater than 1.27 (line D1),the fracture occurs systematically in the base metal, in spite of themodifications of hardenability due to the presence of aluminum in theweld metal zone, and in spite of the slower cooling rate resulting fromincomplete contact between the part and the die. FIG. 6 also shows thatbeyond a ratio of 1.59 (line D2), a particular brittleness occurs.

This maximum ratio of 1.59 between the carbon content of the weld metalzone and the carbon content of the base metal is also obtained bydetermining the critical conditions that lead to the sudden fracture ofa martensitic structure weld comprising a surface defect, when stress isapplied perpendicular to the welding direction.

For this purpose, consideration is given to the case of two sheets 11,12, the thickness w of which is 3 mm, and a slot type defect in the weldmetal zone, the depth of which is 10% of the thickness of the sheets 11,22, i.e. a depth of 0.3 mm.

The expression of the stress intensity factor K_(I) expressed inMPa√{square root over (m)} is as follows:

K _(I) =kσ√{square root over (πa)}

in which

k is the shape factor, and determined in particular on the basis of theratio a/w

σ is the stress applied to the weld, expressed in MPa, and

a is the depth of the defect in question, expressed in meters.

To evaluate the stress intensity factor, a case of severe stress isconsidered, in which the applied stress σ is equal to the elastic limitRe.

Table 2 below expresses the elastic limit Re and the stress intensityfactor K_(I) for four levels of carbon in the weld metal zone varyingbetween 0.2% and 0.4% for a martensitic microstructure.

TABLE 2 0.2% C 0.3% C 0.35% C 0.4% C Re (MPa) 1200 1350 1425 1500K_(I)(MPa{square root over (m)}) 41.3 46.4 49.0 51.6

Reference is made to FIG. 9, which shows the variation of the criticalstress intensity factor K_(w) as a function of the temperature forcarbon contents varying between 0.2 and 0.4% and martensiticmicrostructures. The curve 60 relates to a carbon content of 0.2% C,curve 61 to a carbon content of 0.3% C, curve 62 to a carbon content of0.35% C and curve 63 to a carbon content of 0.4% C.

This FIG. 9 presents the values of the stress intensity factor K_(I)expressed in table 2 for each of the levels of carbon content,identified respectively as 64 for a carbon content of 0.2% C, 65 for acarbon content of 0.3%, 66 for a carbon content of 0.35% and 67 for acarbon content of 0.4%.

The risk of sudden fracture of the weld at −50° C. is thereforeeliminated when the toughness K_(IC) at this temperature is greater thanthe stress intensity factor K_(I).

FIG. 9 shows that this condition is satisfied provided that the carboncontent does not exceed 0.35%.

The result is a maximum carbon content in the weld metal zone of 0.35%.Considering a welded joint fabricated from two sheets of type 22MnB5steel, i.e. containing 0.22% carbon, the limit value of the ratiobetween the carbon content of the weld metal zone and the carbon contentof the steel sheet beyond which there is a risk of sudden fracture inthe weld metal zone is therefore 1.59.

In addition, the fact that the fracture always occurs in the base metalbeyond this value of 1.27 is unexpected, because the toughness of themolten metal decreases as the carbon content increases. Coupled with theeffect of stress concentrations which is unavoidable in the weldedjoint, the fracture should rather have occurred in the molten metal onaccount of a lack of toughness for the highest carbon levels.

For this purpose, the risk of sudden fracture in a weld at −50° C., asdetermined under the conditions specified above, was compared with therisk of sudden fracture at this same temperature in the base metal,where the base metal contained a defect in the thickness of its metalcoating.

The defect in question is a micro-defect 30 μm deep corresponding to thethickness of the metal alloy coating. For a type 22MnB5 steel with thecarbon content of 0.22%, the elastic limit Re is 1250 MPa. If this steelis stressed at a stress level equal to its elastic limit, the stressintensity factor K_(I) is 13.6 MPa. √{square root over (m)}.

By referring to this letter value in FIG. 9 under reference number 68,it can be determined that the sudden fracture should theoretically occurin the weld metal zone and not in the base metal. However, contrary towhat was expected, the inventors found that when the ratio between thecarbon content of the weld metal zone and the carbon content of the basemetal is between 1.27 and 1.59, the fracture systematically occurs inthe base metal and not in the weld metal zone. In summary, the inventorshave found that the increase in the carbon content in this specificrange makes it possible to increase the strength characteristics of theweld metal zone of the hot stamped part, and without any increase in therisk of sudden fracture in this zone, an altogether unexpected effect.

In addition, the inventors have sought to define a simple method todefine the zone claimed by the invention on the basis of the hardnesscharacteristics of the weld metal zone and of the neighboring base metalin the hot stamped part. The significant hardness of the weld metal zoneis linked to its martensitic microstructure, which does not contain anyferrite. It is known that the hardness of a steel with a martensiticstructure is principally a function of its carbon content. Consequently,it is possible to define, on the basis of the above results, the ratio Zbetween the hardness of the weld metal zone and the hardness of theneighboring base metal which must be respected.

In the case of the welding of sheets of different compositions, Cmaxdesignates the carbon content of the sheet that has the highest carboncontent. In the case of welding of identical sheets, Cmax designatestheir carbon content. A fracture in the base metal during theapplication of tensile stress to a welded joint occurs when the ratio Zis greater than a critical value which is a function of Cmax, i.e.1.029+(0.36 Cmax).

For the welding of identical sheets containing 0.22% carbon, a fracturein the base metal is therefore observed when the ratio Z is greater than1.108, i.e. when the hardness of the weld metal zone exceeds thehardness of the base metal by approximately 11%.

With reference to FIG. 7, the curves 47 and 48 represent the evolutionof the microhardness in the weld metal zone and in the neighboring zonesof the welded zone represented on the respective micrographs M1 and M2,for a volume percentage of filler metal of 15% and for differentthicknesses of welded sheets.

For the curve 47, relative to a cooling rate of 30° C. per second, themicro hardness measurements were conducted at the level of the lateraledge of the weld metal zone at one-half the thickness of the thinnestsheet as illustrated in the micrograph M1 by the dotted line X1.

For the curve 48, relative to a cooling rate of 50° C. per second, themicro hardness measurements were conducted at the level of the bottom ofthe weld metal zone at one-half the thickness of the thinnest sheet asillustrated in the micrograph M2 by the dotted line X2.

With reference to FIG. 8, the preferred limit conditions in terms ofpercentage of filler metal and welding speed for the specificcomposition of filler wire defined above and containing 0.7% carbon aredefined by the hatched zone 50.

This zone 50 is delimited by four boundaries 51, 52, 53, 54.

The first boundary 51 defines the lower limit of the percentage offiller metal. The percentage of filler metal must therefore be greaterthan 12% to keep the welded zone from exhibiting mechanical strengthcharacteristics that are too weak.

The second boundary 52 defines the upper limit of the percentage offiller metal. The percentage of filler metal must therefore be less than26%, because above this limit, the welded zone exhibits a brittlenesswhich is incompatible with the required properties.

The third boundary 53 defines the lower limit of the welding speed. Thewelding speed must therefore be greater than 3 meters per minute toobtain a satisfactory geometry of the weld bead and to prevent oxidationphenomena.

Finally, the fourth boundary 54 defines the upper limit of the weldingspeed and is in the shape of a curve.

This fourth boundary 54 is defined on the basis of the experimentalpoints 40, 41, 42 discussed above and for which the experimental points42 correspond to specimens for which the mixing between the filler metaland the base metal is insufficient and/or the weld does not penetrate toa sufficient depth. In addition, the curved shape of this fourthboundary 54 is estimated with reference to requirements specific to thewelding operation.

In fact, the capacity of the laser source to melt the filler wire and tocause a relatively homogeneous mixing has an influence on the maximumpercentage of filler metal and on the welding speed.

For this purpose, for a welding speed of 4 meters per minute, forexample, the percentage of filler metal must not be greater thanapproximately 25%.

For a higher welding speed, the proportion of filler metal must belimited.

In approximation of this fourth boundary 54, the equation of thestraight line 55 that passes through a first point 56 located at thejunction between the upper part of the fourth boundary 54 and the secondboundary 52, and through a second point 57 located at the junctionbetween the lower part of the fourth boundary 54 and the first boundary51 was estimated.

The equation of this straight line 55 is Y=3.86X+39.5 where Y is thepercentage of filler metal and X is the welding speed expressed inmeters per minute.

It can therefore be assumed approximately that the fourth boundarydefining the maximum limit of the welding speed is defined by thestraight line 55 for a welding speed greater than 3.5 m/m.

Therefore, the invention makes it possible to economically fabricatestructural and safety parts for the automobile industry.

What is claimed is: 1-29. (canceled)
 30. A welded steel part obtained byheating in the austenitic range followed by hot forming, then cooling,of at least one welded blank obtained by a butt welding of at least afirst and a second sheet comprising: a steel substrate; and apre-coating including an intermetallic alloy layer and a metal alloylayer of an aluminum or aluminum-base alloy, the intermetallic alloylayer contacting the steel substrate, the metal alloy layer topping theintermetallic alloy layer; a weld metal zone resulting from the weldingoperation and forming a bond between the first and second sheets; themetal alloy layer being removed from edges peripheral to the weld metalzone while the intermetallic alloy layer remains; over at least aportion of the weld metal zone, a ratio between a carbon content of theweld metal zone and a carbon content of the steel substrate of the firstor second sheet having a higher carbon content Cmax, is between 1.27 and1.59; a composition of the steel substrate of at least the first or thesecond sheet, comprises the following elements, expressed in per cent byweight: 0.10%≦C≦0.5% 0.5%≦Mn≦3% 0.1%≦Si≦1% 0.01%≦Cr≦1% Ti≦0.2% Al≦0.1%S≦0.05% P≦0.1% 0.0002%≦B≦0.010%, the balance being iron and unavoidableimpurities from processing.
 31. The steel part as recited in claim 30,wherein the ratio between the hardness of the welded metal zone and thehardness of the substrate of one of the first or second sheets having ahigher carbon content is greater than 1.029+(0.36 Cmax), whereby Cmax isexpressed in per cent by weight.
 32. The steel part as recited in claim30, wherein the composition of the steel substrate of at least the firstor second sheet includes the following elements, expressed in per centby weight: 0.15%≦C≦0.4% 0.8%≦Mn≦2.3% 0.1%≦Si≦0.35% 0.01%≦Cr≦1% Ti≦0.1%Al≦0.1% S≦0.03% P≦0.05% 0.0005%≦B≦0.010%, the balance being iron andunavoidable impurities from processing.
 33. The steel part as recited inclaim 30, wherein the composition of the steel substrate of at least thefirst or second sheet includes the following elements, expressed in percent by weight: 0.15%≦C≦0.25% 0.8%≦Mn≦1.8% 0.1%≦Si≦0.35% 0.01%≦Cr≦0.5%Ti≦0.1% Al≦0.1% S≦0.05% P≦0.1% 0.0002%≦B≦0.005%, the balance being ironand unavoidable impurities from processing.
 34. The steel part asrecited in claim 30, wherein the carbon content of the weld metal zoneis less than or equal to 0.35% by weight.
 35. The steel part as recitedin claim 30, wherein the metal alloy layer of the pre-coating includes,expressed in per cent by weight, between 8 and 11% silicon and between 2and 4% iron, the remainder of the metal alloy layer compositionconsisting of aluminum and unavoidable impurities.
 36. The steel part asrecited claim 30, wherein a microstructure of the weld metal zoneincludes no ferrite.
 37. The steel part as recited in claim 30, whereina microstructure of the weld metal zone is martensitic.
 38. The steelpart as recited in claim 30, wherein hot forming of the welded blank isperformed by a hot stamping operation.
 39. The steel part as recited inclaim 30, wherein the aluminum or aluminum alloy of the metal alloylayer is removed from respective cut edges (of peripheral edges of thefirst and second sheets destined to undergo the welding operation.
 40. Amethod for the fabrication of a welded steel part as recited in claim30, comprising the steps of: providing at least the first and the secondsteel sheet; removing the metal alloy layer from at least one surface ofa portion of the peripheral edges of each of the first and second steelsheets; butt welding the first and the second steel sheets at a level ofthe respective edges of the first and second steel sheets from which themetal alloy layer has been removed with a laser source and using afiller metal wire over at least a portion of the welded metal zone, thefiller metal wire having a carbon content higher than that of the steelsubstrates of at least one of the first or second sheets to obtain awelded blank; heating the welded blank to give the weld metal zone anaustenitic structure; hot forming and heating the welded blank to obtaina steel part; and cooling the steel part at a controlled rate to obtainspecified mechanical strength characteristics.
 41. The method as recitedin claim 40, wherein the metal alloy layer has been removed from each ofthe facing surfaces of the respective peripheral edges of each of thefirst and second steel sheets leaving the intermetallic alloy layer inplace.
 42. The method as recited in claim 40, wherein a width of a zonefrom which the metal alloy layer has been removed at the level of theperipheral edge of the first and second sheets destined to undergo thewelding operation is between 0.2 and 2.2 mm.
 43. The method as recitedin claim 40, wherein a composition of the steel substrate of at leastthe first or second sheet includes the following elements, expressed inper cent by weight 0.15%≦C≦0.4% 0.8%≦Mn≦2.3% 0.1%≦Si≦0.35% 0.01%≦Cr≦1%Ti≦0.1% Al≦0.1% S≦0.03% P≦0.05% 0.0005%≦B≦0.010%, the balance being ironand unavoidable impurities from processing.
 44. The method as recited inclaim 40, wherein the composition of the steel substrate of at least thefirst or the second sheet includes the following elements, expressed inper cent by weight: 0.15%≦C≦0.25% 0.8%≦Mn≦1.8% 0.1%≦Si≦0.35%0.01%≦Cr≦0.5% Ti≦0.1% Al≦0.1% S≦0.05% P≦0.1% 0.0002%≦B≦0.005%, thebalance being iron and unavoidable impurities from processing.
 45. Themethod as recited in claim 40, wherein during the step of butt welding,the peripheral edges of the first and second steel sheets to be weldedare located at a distance of 0.1 mm or less from each other.
 46. Themethod as recited in claim 40, wherein a linear welding energy of thelaser source during the welding operation is greater than 0.3 kJ/cm. 47.The method as recited in claim 46, wherein the laser source is a CO₂ gaslaser type, which delivers a linear welding energy greater than 1.4kJ/cm, or a solid-state laser type which delivers a linear weldingenergy greater than 0.3 kJ/cm.
 48. The method as recited in claim 47,wherein a welding speed is between 3 meters/minute and 8 meters/minute,and a power of the CO₂ gas laser is greater than or equal to 7 kW and apower of the solid state laser is greater than or equal to 4 kW.
 49. Themethod as recited in claim 40, wherein the butt welding step isperformed under a helium or argon cover gas.
 50. The method as recitedin claim 49, wherein a flow rate of helium or argon during the buttwelding step is greater than or equal to 15 liters per minute.
 51. Themethod as recited in claim 40, wherein the filler metal wire comprisesthe following elements, expressed in per cent by weight: 0.6%≦C≦1.5%1%≦Mn≦4% 0.1%≦Si≦0.6% Cr≦2% Ti≦0.2% the balance being iron andunavoidable impurities from processing.
 52. The method as recited inclaim 51, wherein the filler metal wire includes the following elements,expressed in per cent by weight: 0.65%≦C≦0.75% 1.95%≦Mn≦2.05%0.35%≦Si≦0.45% 0.95%≦Cr≦1.05% 0.15%≦Ti≦0.25% the balance being iron andunavoidable impurities from processing.
 53. The method as recited inclaim 52, wherein a proportion of filler metal relative to a volume ofthe weld metal zone is between 12% and 26% and the welding speed isbetween 3 and 7 meters/minute.
 54. The method as recited in claim 53,wherein filler metal relative to the volume of the weld metal zone andthe welding speed are within a range (50) illustrated in FIG.
 8. 55. Themethod as recited in claim 54, wherein a proportion of filler metalrelative to the volume of the weld metal zone is between 12% and 26%,the welding speed is between 3 and 7 meters per minute and when thewelding speed is greater than 3.5 meters per minute, the proportion offiller metal relative to the volume of the weld metal zone and thewelding speed is such that Y≦−3.86X+39.5, whereby Y designates theproportion of filler metal expressed as a volume per cent and Xdesignates the welding speed expressed in meters per minute.
 56. Themethod as recited in claim 53, wherein the proportion of filler metalrelative to the volume of the weld metal zone is between 14 and 16%, ahelium or argon flow rate is between 13 and 17 liters per minute, adiameter at a point of impact on the sheet of the laser beam is between500 and 700 μm, and an extremity of the filler wire is at a distancefrom the point of impact of the laser beam on the sheet of between 2 and3 mm.
 57. The method as recited in claim 40, wherein a cooling rate ofthe weld metal zone during the hot forming step is greater than or equalto a critical martensitic hardening rate of the weld metal zone.
 58. Astructural or safety part for an automotive vehicle comprising: a weldedsteel part as recited in claim
 30. 59. The method as recited in claim40, wherein the steps are performed successively.
 60. The method asrecited in claim 40, wherein the structure of the weld metal zone iscompletely austenitic.
 61. The method as recited in claim 40, wherein onrespective cut edges of the peripheral edges of the first and secondsheets destined to be welded, the metal alloy layer is removed during aprevious cutting operation of each of the first and second sheets. 62.The steel part as recited in claim 30, wherein the welded steel partincludes a mechanical strength greater than 1230 MPa.